This invention relates in general to control systems and, more particularly, to a dual channel actuator for use with an autopilot system to regulate control surfaces of an aircraft.
Conventional autopilot systems for aircraft such as helicopters utilize either series or parallel actuators which serve as the electro-mechanical interface between the autopilot computer and the mechanical control rod. Series actuators generally comprise a motorized jack-screw which is inserted in series or in line with the control rod. Movement of the jack-screw in response to signals from the autopilot computer regulates the control rod and aircraft control surfaces.
Placement of a jack-screw in series with the control rod does not allow the pilot to backdrive the actuator. This presents a serious problem if the autopilot or actuator malfunctions and the jack-screw expands too far. If this should occur, the only way to move the control rod and regain control of the aircraft is to drive the internal actuator motor in the appropriate direction. Malfunction of the actuator, however, would normally prevent operation of the motor, leaving the pilot without an effective means for repositioning the actuator.
The previous solution to this problem has been to limit the full travel range and hence the authority of the series actuator. A typical actuator range has been 10% of the full control range available to the pilot. If an autopilot or actuator malfunction should occur, the pilot would still have 90% of the control range available for manual flight control.
While limiting the travel range of the series actuator may be adequate for certain applications, it is a less than desirable solution from the standpoint of optimum autopilot performance.
Even though the autopilot system can be normalized so that the limited actuator travel provides adequate performance at an average air speed and weight, operation of the helicopter with a limited range of actuator travel at the extremes of its speed and weight flight envelope results in less than optimum flight performance.
Parallel actuators have also been utilized as the autopilot electro-mechanical interface. These actuators typically utilize a motor and spur gear reduction system that allows bi-directional rotational output. A bell crank or bridle cable assembly is often used to couple the actuator with the control rod.
Parallel actuators employ two safety features which are not found on series actuators. An internal disconnect mechanism allows the pilot to disengage the actuator's motor and gear reduction system from the output cable drum. This prevents the actuator from affecting the flight control system and restores complete control authority to the pilot.
An override slip clutch is another safety feature of parallel actuators and allows the pilot to overpower any actuator movement by moving the cockpit control to the desired position. The clutch is contained within the output cable drum and movement of the cockpit control causes the clutch to slip. This allows the output cable drum to rotate relative to the internal gear drive train and restores flight control to the pilot.
Even with the internal disconnect and the override slip features of the parallel actuator, it is still desirable to limit the full travel range or speed of the actuator in the event of autopilot or actuator malfunction. As with series actuators, restrictions on the authority of parallel actuators result in less than optimum autopilot performance when the aircraft is operated at the extremes of its flight envelope.